1. Field of Invention
The present invention is concerned with an instrument for measuring the photosynthetic activities and capacities of plants. More particularly it is concerned with an instrument for measuring simultaneous chlorophyll fluorescence emission, CO.sub.2 assimilation rate and, if desired, transpiration rate, parameters which are all characteristics of a plant's general metabolic and health status.
Recent demonstrations that changes in chlorophyll fluorescence emission during the induction of photosynthesis are closely related to the rate of carbon dioxide assimilation have increased the interest in the application of fluorescence as a convenient monitor of leaf photosynthetic activity.
2. Description of Prior Art
Apparatus and techniques for measuring chlorophyll fluorescence emission from leaves in the laboratory are known. Chlorophyll fluorescence emission is indicative of the light reactions of photosynthesis, and is the first detectable parameter to indicate a change in photosynthetic activity. It has been found that the level of photosynthetic activity of the leaf as evidenced by the fluorescence emission can provide not only evidence of the general health of the leaf but also how the leaf responds to changes in its environment. The response may be rapid and provide an indication of the leaf's activity well in advance of any visible physical change in the leaf.
Simultaneous measurements of CO.sub.2 assimilation rate of a leaf and continuous fluorescence signals have previously been made under laboratory conditions. To date, apparatus for measuring CO.sub.2 assimilation rate and the fluorescence signals have employed the continuous means of using white light fitted with a red-removing filter, so that continuous fluorescence induction curves may be generated by the red-deficient light, and by detecting the fluorescence of the leaf, which is emitted as red light, with a continuous detector fitted with suitable filters so that only red light, all of which is derived from fluorescence of the leaf is detected. This method of detecting fluorescence gives a measure of the fluorescence induced when light deficient in the red portion of the spectrum is incident upon the leaf.
In field conditions white light contains red light ie the part of the spectrum which would be eliminated by the red removing filter. Some of the light, which would not be present in the red-deficient light is capable of driving photosynthesis. The continuous method described above is not representative of the activity of the plant when white light is incident upon it, as under field conditions. Therefore, it was desirable to develop a system for measuring the fluorescence response of a leaf which is exposed to white light, simultaneously with the measurement of CO.sub.2 assimilation by the leaf.